An RFID system using an electric wave has a long communicable distance (several tens centimeters to several meters) for a passive-type non-contact IC tag without power source. However, the communicable distance for the non-contact IC tag is not constant, because it is easily changed due to some factors, such as a direction of the antenna mounted on the non-contact IC tag, etc. Further, the communicable distance has a characteristic of a non-continuous distribution (an information readable section and non-readable section) due to an effect of reflection of an electric wave or the like. Thus, while the actual communicable area is widely expanded, its boarders cannot be clearly defined, and may remain vague.
Due to the vagueness of the communicable area, some system is required to sequentially read tags attached onto articles on a belt conveyer, so as to correctly identify the order of the tags or the movement state. In such a system, to avoid communicating with any non-target tags, many restrictions and steps are needed. For example, it is necessary to control intervals between the tags, limit the antenna output, and physically shield the electric wave (cover each article to be read, using an electric shielding curtain). This results in a difficulty on the operations.
Patent document 1 discloses an antenna for tag communication, which can cover a wide communicable area without non-communicable areas with some antennas having high directivity.
Patent document 1: JP2006-020083 A